Aluminum alloys and advanced high strength steels are becoming increasingly common as materials used in automotive body construction. One of the major barriers to wider implementation of these materials is their inherent lack of formability as compared to mild steels. Incorporating lightweight materials such as advanced high strength steels (AHSS) and aluminum alloys (AA) into high-volume automotive applications is critical to reducing vehicle weight, leading to improved fuel economy and reduced tailpipe emissions. Among the most significant barriers to the implementation of lightweight materials into high-volume production are stamping issues and the lack of intrinsic material formability in AHSS and AA.
Numerous stamping challenges are associated with the implementation of AHSS and AA in automotive production. The primary method of stamping body panels and structural parts is forming sheet material between a sequence of two sided dies installed in a transfer press or a line of presses. During the era of low oil prices, most automotive parts were stamped from Deep Drawing Quality (DDQ) steel or even Extra Deep Drawing Quality (EDDQ) steel, with both alloys exhibiting a maximum elongation in plane strain above 45%. The formability of aluminum alloys, on the other hand, typically does not exceed 25%. In practice, stamping engineers do not intend to form sheet metal beyond a level of 15% in plane strain due to the much lower work-hardening modulus of metals in these strain ranges, and also due to the danger of local dry conditions on the blank surface. The formability of AHSS is typically around 30%. Insufficient formability drives the necessity to weld difficult to form panels from several parts or to increase the thickness of the blank used in forming the panels.
Electrohydraulic forming (EHF) is a process which can significantly increase sheet metal formability by forming a sheet metal blank into a female die at high strain rates. The high strain rate is achieved by taking advantage of the electrohydraulic effect, which can be described as the rapid discharge of electric energy between electrodes submerged in water and the propagation through the water of the resulting shockwave—a complex phenomenon related to the discharge of high voltage electricity through a liquid. The shockwave in the liquid, initiated by the expansion of the plasma channel formed between two electrodes upon discharge, is propagated towards the blank at high speed, and the mass and momentum of the water in the shockwave causes the blank to be deformed into an open die that has a forming surface. The shockwave forces the blank into engagement with the forming surface to form the metal blank into the desired shape.